1. Field of the Invention
The present invention relates to smoke detectors in which the presence of smoke is sensed by directing a beam of light into the smoke and sensing the light scattered. The invention also relates to dark field optical systems in which light scattering is reduced to darken the field. The invention also relates to the design of illuminating and light collecting optical elements.
2. Description of the Prior Art
Smoke detectors available for home use currently fall into two categories. Those which are of the ionization type, and those which are of the optical type. The present invention deals with a smoke detector of the optical type.
The conventional optical smoke detector contains a light source which illuminates a sample of air potentially containing smoke. If smoke is present, light is scattered in all directions from the scattering particles. In known optical systems, the collection is at an off-axis position to one side of the beam. It is known that the scattering flux is very much a function of the scattering angle. For instance, back scattering is relatively weak, side scattering reaches a minimum value, and forward scattering is relatively strong. The scattering efficiency varies through nearly two orders of magnitude as one goes from side scattering (perpendicular to the beam) to forward scattering in substantial alignment with the beam. Since this principle has been known, a system has been proposed in which the main illuminating beam is obscured by a stop and light is collected by a lens placed off the illuminating axis, behind a stop. In such off-axis systems, the scattered light collection solid angles are small and the light detection efficiency low since only a small part of the scattered light is collected.